1. Field of the Invention
This invention relates to an improvement in the method which prevents the deterioration of a hardened mass of e.g., concrete containing a corrosive metallic material, or an aggregate which is reactive with an alkaline substance, by preventing the corrosion of the metallic material or the reaction of the aggregate with any alkaline substance.
2. Description of the Prior Art
A hardened mass of concrete has numerous pores opening to its surface. It is known that a steel reinforced concrete structure undergoes deterioration outdoors with the passage of time until its surface cracks. More particularly, it is known that the carbon dioxide and water which the air contains enters the concrete through its pores and causes its neutralization and the rusting of steel reinforcing or frame members, and that the expanding rust exerts pressure on the surrounding concrete until the latter eventually cracks.
It is also known that the aggregate which concrete contains reacts with an alkaline substance. This phenomenon also causes an outdoor concrete structure to crack with the passage of time, particularly when it contains as the aggregate crushed stone which has come to be used to make up for the insufficiency of good gravel. The reaction forms a network or tortoise-shell pattern of cracks on the concrete surface.
In either event, the cracking of a concrete structure is most likely to result in its destruction and must, therefore, be prevented as far as possible. It is also highly desirable to estabilish a repairing method which can effectively prevent the progress of deterioration of an existing concrete structure.
Japanese Patent Laid-open Publication No. Sho 60-108385 (108385/1985) teaches that as a method of preventing the deterioration of concrete containing a corrosive metallic material, such as reinforced concrete, it is effective to impregnate its surface with an aqueous solution of nitrite and lithium silicate. Japanese Patent Laid-open Publication No. Sho 61-256951 (26951/1986) teaches that as a method of preventing the reaction of the aggregate in concrete with an alkaline substance, it is effective to impregnate the surface of the concrete with litium hydroxide, lithium nitrite, etc.
Japanese Patent Laid-open Publication No. Sho 63-21270 (21270/1988) discloses an improved method that after a rust proof agent is impregnated from the surface of concrete, epoxy resin is applied on the surface of the concrete.
These methods are, however, far from satisfactory. If an aqueous solution of lithium nitrite is used as the impregnant as taught by Japanese Patent Laid-open Publication No. Sho 61-256951 (26951/1986), the water contained in the concrete is hardly evaporated upon the drying of concrete due to the hygroscopicity of nitrite itself. It if is allowed to dry in the air, its full drying may require a period of as long as several months and it may not be very rare that dew forms on its surface. If its surface is wetted again, as by rainfall, the nitrite may be dissolved in rainwater and fail to provide a long period of protection for the concrete.
If an aqueous solution of nitrite and lithium silicate is used as taught by Japanese Patent Laid-open Publication No. Sho 60-108385 (108385/1985), the silicate forms during the drying of the impregnated concrete an irreversible gel which closes the pores of the concrete and does not allow the nitrite to be readily dissolved in rainwater falling on the concrete surface. Even in this way, however, it is impossible to prevent the dissolution of the nitrite completely, particularly when the concrete has only a small depth of impregnation below its surface.
Japanese Patent Laid-open Publication No. Sho 63-21270 (21270/1988) proposes an improved method in which the surface of concrete which has been impregnated with an aqueous solution of lithium nitrite is coated with an epoxy resin. The epoxy resin, however, prevents the dissolution of the nitrite only until its own deterioration occurs. Therefore, the proposed method is not very effective for protecting concrete against deterioration over a long period of time. Moreover, the method is not recommneded for practical use, since the epoxy resin is expensive and its application to the concrete surface means an additional job.